1. Field of the Invention
The invention relates to metallization of ferrites and, more particularly, to metallization of ferrites by surface reduction with a gaseous reducing agent.
2. Description of the Related Art
Ferrites, a class of oxide ceramic materials based on Fe.sub.2 O.sub.3, have a variety of useful electrical and magnetic properties which make them attractive for devices such as inductors and transformers. Metallization of ferrite surfaces is often needed to prepare electrically conductive paths for circuit elements. Strong adhesion of the metallized layer is desirable for device durability and reliability. When further processing is required following metallization, adhesion of the metallized layer the ferrite must be sufficiently high to withstand the additional stress induced, e.g., by processes such as baking or electroplating.
Generally, ceramic metallization processes fall into three categories: thin-film, thick-film, and co-firing techniques. In the thin film approach, a thin layer of metal is deposited by vacuum processes such as sputtering, evaporation, chemical vapor deposition, and laser ablation. Electroless and electrolytic plating are also frequently grouped in the thin film category. To enhance adhesion, a preliminary adhesion-promoting layer, such as chromium or titanium, is often deposited.
Thick film methods involve screen printing metal pastes, typically metal powders mixed with glass frits and organic binders, onto ceramic substrates. The printed substrates are fired to form conductive paths on the ceramic. In the co-firing approach, unfired "green" ceramic substrates are coated with patterned metal paste lines. The printed green substrate is fired both to sinter the substrate and form the conductive metal patterns.
These processes have several disadvantages. Thin film techniques such as sputtering and electron-beam evaporation require vacuum deposition equipment. Thick film and co-firing methods generally employ precious metals such as silver and/or palladium. High temperature processing causes dimensional changes and can create stresses due to differing coefficients of thermal expansion for printed substrates containing dissimilar materials.
Other approaches to ceramic metallization are disclosed in U.S. Pat. Nos. 4,663,826 and 5,091,820. In the '826 patent, a region of increased conductivity is generated by laser irradiation in a reducing atmosphere. In the '820 patent, electodes are formed on opposing faces of a piezoelectric cylinder through reduction followed by selective removal of reduced layer portions.
In view of the difficulties and high costs associated with conventional metallization processes, there is need in the art for simple, low-temperature processes to create conductive paths on ceramic substrates. More particularly, there is a need in the art to create adherent metallization on ferrite surfaces to define circuit elements and magnetic component elements for ferrite devices.